Printing ink is a complex mixture of ingredients that are combined in a specific formulation to meet desired characteristics of the printing application of the ink. It is well-known that printing inks utilized today are typically modified with various types of additives to provide required rheological properties to ensure quality printing on high-speed printing presses. Generally, polymer (or resin), solvent, and pigment types determine the intrinsic rheology of the ink formulation. It has been known that clays can be used to control rheology and reduce ink misting on high-speed presses.
Offset color ink is generally formulated with various amounts of an extender pigment to achieve the rheological requirements of the process. Clays are often used for this purpose since they are low in cost and have the ability of decreasing the misting and slinging of the inks when printed at high speeds. The amount of clay used in such applications varies from about 2-10 wt % for letter press color inks and from 10-25 wt % for web offset color inks. Heatset ink formulations presently can include up to about 5 wt % clays, but at higher loadings there is a negative impact on ink gloss.
The incorporation of clay into the manufacture of inks and the larger content of clay required in web offset inks cause two basic problems. First, the clay presents difficulty in dispersing itself in the ink if the ink is manufactured using high-speed mixing without roller milling. However, clays have a hydrophilic surface and consequently are very difficult to be wetted and dispersed by hydrocarbons such as mineral oils and varnishes. In addition, dispersions of conventional clays and hydrocarbons are not very stable on aging, and some agglomeration of clay particles occurs.
The second problem occurs during printing of lithographic inks (offset inks) whereby the clay in the ink is leached by the fountain solution and slowly migrates in it. The hydrophilicity of the clays is such that the clay is extracted from the ink and begins to dissolve into the water together with traces of color pigments. This causes a bleeding problem. In addition, the presence of clay in the fountain solution causes a variety of printability problems resulting in inadequate reproduction of colors.
There is a commercial need for modified clays to increase viscosity and maintain gloss in lithographic inks, where the clays wet easily with hydrocarbon solvents and do not leach by the fountain solutions. What is particularly needed is a surface-enhanced clay for use in heatset and sheet-fed inks. It is desired to replace ink resin and ink oil with minimal impact on key ink properties such as ink gloss and ink densities, with little or no ink bleeding to fountain solution even at high ink clay loadings.